This invention relates to thermoset epoxy compositions for adhesively bonding molded panels of sheet molding compund to one another. The invention relates particularly to adhesives for molded SMC that cure rapidly and retain their adhesive strength at elevted temperatures.
In an effort to reduce the weight and corrosion susceptibility of automotive body panels, there has been a move towards making them from sheet molding compound (SMC). SMC is a moldable composite material generally made up of an unsaturated liquid polyester resin, a low profile thermoplastic resin, an inert filler, magnesium oxide or other maturation aids, and shaped or milled glass fiber reinforcement. The constituents are generally mixed between two protective films of polyethylene or other such material. The SMC sandwich is then allowed to age until the polyester resin matures to a putty-like consistency. Preliminary to molding, the protective sheets are removed and the matured SMC is cut into pieces of appropriate size for a particular mold. Parts are formed from SMC by laying these pieces at suitable locations in a compression mold. The mold is closed and the resinous material flows under the applied heat and pressure. Heating initiates final cross-linking of the polyester resin and the incumbent formation of a self-supporting fiber reinforced SMC part.
SMC has been used in the automotive industry for some time. For example, the Corvette model manufactured by the Chevrolet Division of General Motors has molded SMC body panels. It is often preferable to adhesively bond such panels to each other or to underlying support members rather than mechanically attaching them. However, when SMC panels are used in conjunction with metal panels on the same vehicle body and painted at the same time, it is necessary for the adhesive to tolerate the same conditions as the metal in the finishing process. Some automotive finishes for metals require paint oven temperatures in the range of 180.degree. C. While this presents no problem to SMC panels themselves, prolonged heating in paint ovens generally causes ffailure or substantial degradation of known adhesives for SMC. The most commonly used SMC adhesives today are two part polyurethane materials. Although these adhesives cure rapidly, at elevated temperatures their bond strength is substantially diminished. In fact, all of the commercial adhesives that we tested failed at 190.degree. C. or less. Most experienced catastrophic losses in bond strength even at temperatures around 160.degree. C.: that is, the adhesion bonds failed in the ovens before the paint cured.
Another production consideration is the time required for the adhesive to cure. These times should be short (about three minutes or less) to be compatible with assembly line operations. The bond strength after this initial cure must be great enough to support the weight of a panel without special fixturing. While urethanes meet this criteria, as noted above, they cannot take high temperatures.